I. Field of the Invention
The present invention relates to battery chargers. More particularly, the present invention relates to a novel and improved method and apparatus for rapid charging of batteries that compensates for sub-optimal interconnection between the charger and the batteries.
II. Description of the Related Art
The availability of radio frequency (RF) spectrum has led to the proliferation of wireless communication devices. One primary advantage of wireless communication devices is portability. Wireless communication devices can be used without being tethered by power cords or wire line links. The wireless communication device portability imposes a requirement for a portable power source. Batteries supply the portable power source for wireless communication devices.
Many types of battery designs are available. The choice of a particular battery type is determined by the application. Factors that contribute to the determination of battery type include required device supply voltage, peak current capability, storage capacity, recurring costs, and physical constraints. For a wireless communication device such as a wireless phone, a rechargeable battery is required.
There are three basic types of rechargeable batteries that can satisfy the design constraints imposed on a wireless phone battery. The three battery types are Nickel Cadmium (NiCad), Nickel Metal Hydride (NiMH), and Lithium Ion (Li-Ion). Each battery type has inherent advantages and disadvantages.
The NiCad battery is the least expensive and the most common rechargeable battery type. A single NiCad cell has a voltage of approximately 1.2 volts. A number of individual NiCad cells are stacked in series in order to achieve the required wireless phone supply voltage. The NiCad battery is fairly forgiving with respect to the voltage tolerance of the charger. However, NiCad technology presents two disadvantages. The capacity of a NiCad battery is lower than a NiMH or Li-Ion battery of the same size or weight. Moreover, the NiCad battery suffers from capacity degradation if subject to numerous recharge cycles where the battery was not fully discharged prior to recharging. This memory effect is particularly troublesome in a wireless phone application since the user typically does not fully discharge the battery before recharging it. Rather, the user typically will replace the battery within the charger at the end of each day regardless of the amount of use the phone incurred during the day. This routine is typically followed to insure the user has a fully charged battery at the start of each day. Starting each day with a fully charged battery minimizes the probability that communication will be lost or that a connection cannot be made due to a lack of battery power. Unfortunately, the routine of recharging a partially discharged battery at the end of each day maximizes the degradation of the capacity of a NiCad battery due to memory effects.
The NiMH battery does not suffer from memory effects when recharged prior to full discharge. The cost of a NiMH battery is higher than that of a NiCad battery but the energy density of a NiMH battery is slightly higher than that of a NiCad battery. Like the NiCad battery, a single NiMH cell has a voltage of approximately 1.2 volts. To achieve the required supply voltage of a wireless phone a number of cells must be stacked in series. The NiMH battery has been used as an alternative to the NiCad battery in wireless telephones.
The Li-Ion battery is presently the most costly rechargeable battery used in wireless phone applications. However, this disadvantage is more than offset by the many advantages a Li-Ion battery offers over the other two battery types. A Li-Ion battery does not suffer from capacity degradation due to memory effects when recharged prior to full discharge. Additionally, the Li-Ion battery has the highest energy density of the three rechargeable battery types discussed. The energy density of the Li-Ion battery is nearly twice that of a NiCad battery. This is especially important in wireless phone applications where there is an emphasis on increased phone talk and standby times with a corresponding emphasis on small size and light weight. A single Li-Ion cell has a cell voltage of approximately 4.1 volts. The high cell voltage eliminates the need to series stack multiple Li-Ion cells to achieve the supply voltage required in a wireless phone.
Regardless of the rechargeable battery chemistry used, a battery pack is typically custom designed for a particular wireless phone application. Because of the stringent size constraints put on wireless phones a standardized battery pack configuration is not feasible. Each battery pack is designed according to the form factor allocated within a particular wireless phone.
The corresponding battery chargers are custom designed for each wireless phone application. The battery charger is designed such that a battery can be recharged while housed within the phone. An additional slot is often provided to allow a battery pack to be charged simultaneously. This allows the user to have a fully charged spare in case the battery installed in the phone becomes fully discharged and the user cannot wait for a complete recharge cycle.
The battery recharge time is largely a function of the battery charger design. A battery charger is a Constant Current-Constant Voltage (CC-CV) power supply. A CC-CV power supply is able to operate in either constant current or constant voltage mode. The power supply provides power to the battery until either a current limit or voltage limit is reached. When a discharged battery is initially placed in the battery charger the voltage of the battery is extremely low. As a result, the power supply in the charger will reach its current limit before it reaches its voltage limit. The voltage of the power supply then varies to maintain the constant current into the battery. Once the battery becomes sufficiently charged, the power supply in the charger is able to achieve the voltage limit. The voltage from the power supply then remains constant and the current from the supply decreases as the battery continues to charge. This condition is maintained until the battery becomes fully charged, at which point a constant voltage is maintained at the battery but effectively no current is sourced into the battery.
One issue addressed in designing the power supply within a battery charger for a wireless phone is the output voltage. All three battery chemistries are available for some models of wireless phones. Since both the NiCad and NiMH batteries utilize series stacked cell structures the output voltage from these batteries is determined by the number of cells combined in series. A typical wireless phone battery pack will utilize three NiCad or NiMH cells in series for an output voltage of 3.6 volts. Note however that a single Li-Ion cell has a charged cell voltage of 4.1 volts. Therefore a battery charger that can charge all three battery chemistries needs to provide at least a 4.1 volt output voltage. Fortunately, both the NiCad and NiMH batteries have moderate over voltage tolerance and are able to withstand a 4.1 volt charging voltage. On the other hand, a Li-Ion battery has very low over voltage tolerance. The power supply charging a Li-Ion battery must maintain the ultimate charging voltage to a tight tolerance. The ultimate charging voltage or top off voltage used on a Li-Ion charger is especially important because the life and capacity of the Li-Ion cell is degraded if the top off voltage varies from 4.1 volts by more than a few percent.
In addition to the charging voltage limit, the battery charger must provide a reasonable charging current limit. The three rechargeable battery chemistries have different current limitations. The current limit of the charger is designed to be below the safe operating point of each battery type. The designed current limit for typical wireless phone battery chargers is approximately one amp.
Although the battery charger may produce a 4.1 volt top off voltage the actual voltage at the battery may be significantly less than 4.1 volts for a majority of the charging time. This is due to a combination of a number of factors.
The internal configuration of the battery pack is a contributor to the charger/battery voltage differential. Most wireless phone battery packs include some form of protection circuit within the battery. The protection circuit guards against overvoltage, and provides current limiting to the battery. However, any series element within the battery pack between the battery and the charger contributes to I*R (current*resistance) voltage drops between the charger and the actual battery cell. Another contributor to voltage drop within the battery pack is the wire or circuit board connecting the battery to the charging terminals. The resistance of this length of conductor will also contribute to I*R losses.
If the battery is charged while housed within the wireless phone, the wireless phone may also contribute to voltage drops between the charger and the battery. Additional losses will occur if the battery is charged through a connector provided on the phone but no additional losses will occur if the battery is charged through the same terminals as would be used when charging the battery pack alone. An example of a condition where the phone contributes to I*R losses is where the battery is charged through a hands free car kit connection on the phone. An example where the phone does not contribute to I*R losses is where the charging terminals on the battery pack are exposed whether or not the battery pack is installed within a phone. If a battery charger utilizes these terminals whether or not the battery pack is installed within a phone, the phone will not contribute to any I*R losses.
The battery charger itself may contribute I*R losses if the charger power supply is connected to the charging terminals using wires or a circuit board. Any series element between the output of the power supply and the charging terminals will produce an I*R loss.
Finally, I*R losses will occur because of the imperfect connection between the terminals on the charger and the terminals on the battery pack.
What is required is a battery charger design that compensates for the losses between the charger power supply and the actual batteries. This will enable the battery to be charged at a faster rate without any detrimental effects on battery cycle life. Compensation of charging losses benefits the user by allowing discharged batteries to be recharged in a shorter period of time.